I’m not sure whether the target audience for James Cameron’s Deepsea Challenge 3D is aficionados of undersea adventure or aficionados of James Cameron.

The movie, which documents the filmmaker’s 2012 expedition — in his own personal submarine — to the deepest point in all of Earth’s oceans, features less 3-D footage of sea cucumbers and exotic octopi than of Cameron’s nose. It’s a movie about exploring the vast “dark continent” of the ocean’s deepest places (to quote Cameron, who produced and narrates the film) that ends up feeling claustrophobic. Much of it was shot inside a metal sphere the size of a fitness ball.

That’s because that is where Cameron is for much of the movie. He paid for the submersible, christened the Deepsea Challenger, so of course he gets to ride it to the bottom of the Pacific’s Mariana Trench. He also gets to take all the risk. At nearly 36,000 feet below sea level, the trench is the deepest known point on the planet, and it hadn’t been visited by a manned vehicle since 1960, when Jacques Piccard and Don Walsh piloted the bathyscaphe Trieste to the spot known as Challenger Deep. The trip is very dangerous, but, admittedly, also pretty exciting.

The Titanic director is well known for his underwater exploits, both on screen and off, having visited (and documented) the site of the Titanic more than 30 times and having made the deep-sea thriller TheAbyss in 1989. Along with footage of those projects, the movie features a dramatized prologue showing the filmmaker playing, as a boy, in a cardboard box that he imagines to be a submarine.

Despite the personal nature of those scenes, the movie makes it clear that Cameron’s interest in financing a voyage to the bottom of the sea is more than egotism. Scientific advancement is the true goal, in the form of knowledge about how tsunamis are formed and in the discovery of new species of deep-sea life forms.

Yet the film pays scant attention to this stated goal, relegating the discoveries achieved by Cameron’s mission to a brief on-screen title at the very end of the film. Even a scene in which Cameron is shown collecting sediment and rock samples seems perfunctory, as though the work is secondary to how cool it is to be floating around a few miles underwater, manipulating a couple of 3-D cameras on robotic arms with joysticks.

Speaking of which, those robotic arms give out at one point, having sprung leaks in their hydraulic conduits. Why did this happen? And why was it not anticipated? And what is that scary cracking sound that we hear during one of Cameron’s early practice dives, that sounds like the filmmaker, to use his own words, is about to get “chummed into a meatcloud in about two microseconds” by an implosion of the hull?

The movie doesn’t tell us. It also isn’t clear whether the deaths of the film’s co-director, Andrew Wight, and cameraman Michael deGruy, in a 2012 helicopter crash, were directly related to the filmmaking or incidental. Cameron seems certain, however, that Wight and deGruy “would have wanted” him to continue.

Maybe they would have. What is certain is that, even after that tragedy, Cameron has a movie — and a message — to deliver: His heart, if nothing else, will go on.

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